Second order Active Filter issue

Discussion in 'Analog & Mixed-Signal Design' started by Aagash, Jun 28, 2017.

  1. Aagash

    Thread Starter Member

    Jun 7, 2017
    43
    2
    When I fed a signal with Vp ~ 120mV with frequency of 1.6Hz to an active second order low pass filter (cutoff freq = 5Hz) , then the resulting signal is more distorted . (used in medical acquisition)
    I'm using LM318 op amp with slew rate of 17V/us (sufficient enough) .
    The values of R = 6.8k , C= 4.7u . Why this distortion takes if the signal is filtered ?
    ActiveFilter.jpg

    Filtered :

    Filtered.JPG

    Unfiltered :

    UnFIltered.JPG Could anyone say the possible solution for this issue . Thanks in advance !
     
  2. crutschow

    Expert

    Mar 14, 2008
    20,223
    5,718
    You schematic is incomplete.
    Show us the actual circuit you built.
    What is the IC power?
    You need a separate plus and minus supply for that circuit.
     
  3. Aagash

    Thread Starter Member

    Jun 7, 2017
    43
    2

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    Attached the sample LTspice pic , but the real time acquisition circuit has not an exact sine wave at input. Even-though it works fine in simulation, in hardware it fails.

    LowPass.PNG

    Datasheet of LM318:
    http://www.ti.com/lit/ds/symlink/lm318-n.pdf
     
  4. crutschow

    Expert

    Mar 14, 2008
    20,223
    5,718
    You output signal looks like a noisy version of the input signal, so it would appear the filter circuit is not working.
    Are you sure you have separate plus and minus power (two supplies) to the respective pins on the op amp?
    What is the power source?

    What does the output look like with a 10Hz sinewave input from a signal generator?
     
  5. crutschow

    Expert

    Mar 14, 2008
    20,223
    5,718
    For such a low frequency, low level signal it might be better to use a lower bandwidth, low noise op amp, such as the NE5532, TL071-4, MC33174, or MC34074.
     
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